1. Field of the Invention
The invention relates to the squeeze casting of metal articles and particularly, although not exclusively, to the squeeze casting of metal pistons for engines or compressors.
Squeeze casting is a process in which a molten metal is placed in a die and is then solidified under a force of many tons in order to produce an article which is of greater strength than conventional gravity cast articles by the elimination of voids and the production of a particularly homogeneous structure. The squeeze casting of pistons of engines or compressors is particularly desirable because of the improved properties which it gives to such pistons.
2. Review of the Prior Art
In general, a squeeze casting die has sides, a bottom and a top die part, with the top die part being movable from a retracted position, to allow molten metal to be poured into the die, to a position in which the top die part closes the die and applies the squeeze force to the solidifying metal. Such dies have a problem, however, in that the interior of the die is not readily accessible both for removal of the article after squeeze casting and, where required for the insertion into the die of inserts or other reinforcements around which the article is to be squeeze cast.
In order to overcome this problem, it would be necessary to form the sides of the die in two or more parts at least one of which is movable away from the die-forming position in order to give access to the interior of the die. There is a problem, however, with such an arrangement because the squeeze casting pressure of many tons applied by the top die part during casting tends to separate such side die parts and so split the die.
One attempt to overcome this problem is shown in U.S. Pat. No. 3,120,038 where the sides of the die are formed by two side die parts carried on respective arms which are pivotable about respective parallel horizontal axes. The arms are pivoted downwardly into alignment to inter-engage the two die side parts. The molten metal is allowed to solidify partially before the pressure is applied and the arms and the pivots can thus take the load imposed on the die side parts by the applied pressure to prevent the side die parts splitting.
It is a disadvantage of this arrangement that partial solidification is necessary before pressure is applied so that the full benefits of squeeze casting are not obtained. In addition only limited separation of the side die parts is available because only limited pivotal movement (about 20.degree.) of the arms is provided. This makes access to the interior of the die difficult, for removal of the squeeze cast article, for the insertion of any insert or reinforcement into the die, for cleaning of the die and for applying a parting medium to the interior surfaces of the die. In addition, this limited opening will not, in some cases, allow the die parts to be formed with projections or recesses because of the difficulty of disengaging such projections or recesses from the cast article as a result of the limited opening of the die parts.